Hybrid integration using folded Mach-Zehnder modulator array block

ABSTRACT

An apparatus comprising a modulation block comprising a plurality of modulators, wherein each of the plurality of modulators comprises an optical input port and an optical output port, and wherein all of the optical input ports and all of the optical output ports are positioned on one face of the modulation block. Another apparatus comprising a modulation block comprising one or more Mach-Zehnder modulators (MZMs), wherein each MZM is coupled to an optical input port, an optical output port, and at least one electrical trace, wherein all of the optical input ports and all of the optical output ports are positioned on a first side of the modulation block, and wherein all of the electrical traces are positioned on a second side of the modulation block, and a planar lightwave circuit (PLC) coupled to the modulation block via an optical interface.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Conventional dense wavelength division multiplexing (DWDM) systemsemploy a laser in combination with a series of modulators, which mayserve the function of manipulating the amplitude, phase, and/orfrequency of the laser to create an optical signal based on a receivedelectrical signal. Multiple modulators may be employed to multiplexseveral different electrical signals, such as radio frequency (RF)signals, onto a single optical signal by employing each modulator ormodulator pair, depending on implementation, to alter a specificwavelength of light. The resulting optical signal may then be placed onan optical fiber for transmission.

One approach to creating a transmitter for use in a DWDM system is touse discrete modulators. However, this implementation method may need acomplex and expensive transmitter architecture encompassing a largenumber of optical functions and elements. This complexity problem hasled to research into large-scale photonic integration, which would allowthe integration of multiple optical functions and wavelength channelsonto a single device or block (sometimes also referred to as a circuitcomponent). The integrated approach may reduce architecture complexity,increase system reliability, and reduce system power consumptioncompared to systems employing discrete optical components.

A Mach-Zehnder modulator (MZM) based on a Mach-Zehnder interferometermay be a key component for high-speed optical transmitters and isfrequently used in DWDM networks. The MZM may be made up with waveguidesin a good optical material, whose refractive index may be altered byapplying an electric field on one or two arms of the interferometer.Intensity and/or phase modulation may be obtained by creating phasedifferences between two arms. Depending on the overall phase difference,the output light recombines more or less efficiently, or does notrecombine at all, at the output of the interferometer, conducting to amodulation of the output power. The MZM may be combined with acontinuous wavelength (CW) laser in a transmitter to allow precisemanipulation of optical signals in both amplitude and phase. As theadvent of coherent optical communications, MZM may become more importantas it is a reliable modulating device capable of supporting coherenttechnologies.

Traditional MZMs may typically be made of lithium niobate (e.g.,LiNbO3). However, lithium niobate is highly anisotropic, which requiresits input ports to be positioned in the opposite direction from itsoutput ports. Further, the anisotropic nature of a traditional MZM mayprevent significant on-chip waveguide bending. The design constraintshave forced all previous attempts at large scale photonic integration torely on a monolithic architecture. The monolithic structure may requirerouting of incoming electrical signals related to the MZMs at the centerof the monolith, which may become very complex given a relatively highnumber of MZMs integrated. This in turn creates an upper limit on thenumber of channels available to a monolithic system. In high-speedsystems, the difference in length between the simple short routing tothe outer MZMs when compared to the longer complex routings to the innerMZMs may also cause a difference in the propagation of the associatedelectrical signals. The difference in propagation delay needs to beconsidered by other components of the system, creating further systemcomplexity and potential increase in cost.

SUMMARY

In one embodiment, the disclosure includes an apparatus comprising amodulation block comprising a plurality of modulators, wherein each ofthe plurality of modulators comprises an optical input port and anoptical output port, and wherein all of the optical input ports and allof the optical output ports are positioned on one face of the modulationblock.

In another embodiment, the disclosure includes an apparatus comprising amodulation block comprising one or more MZMs, wherein each MZM iscoupled to an optical input port, an optical output port, and at leastone electrical trace, wherein all of the optical input ports and all ofthe optical output ports are positioned on a first side of themodulation block, and wherein all of the electrical traces arepositioned on a second side of the modulation block, and a planarlightwave circuit (PLC) coupled to the modulation block via an opticalinterface.

In yet another embodiment, the disclosure includes a method implementedby an optical modulator comprising receiving an optical signal via anoptical input port in a first direction, receiving an radio frequency(RF) input signal, modulating the optical signal using the RF inputsignal to generate a modulated optical signal, and transmitting themodulated optical signal via an optical output port in a seconddirection, wherein an angle between the first and second directions isgreater than 135 degrees.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of an optical modulationblock.

FIG. 2 is a schematic diagram of an embodiment of an optical assembly.

FIG. 3 is another schematic diagram of an embodiment of an opticalassembly.

FIG. 4 is a schematic diagram showing an electrical interface and anoptical interface between a modulation block and a PLC carrier.

FIG. 5 is another schematic diagram showing an interface between amodulation block and a PLC carrier.

FIG. 6 is another schematic diagram showing a cross-sectional view of anassembly.

FIG. 7 is yet another schematic diagram showing a cross-sectional viewof an assembly.

FIG. 8 is a schematic diagram showing an optical interface between afirst waveguide and a second waveguide.

DETAILED DESCRIPTION

It should be understood at the outset that, although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Disclosed herein are apparatuses and methods for large-scale hybridphotonic integration. In embodiments of this disclosure, a modulationcomprising a plurality of MZMs may be constructed using a Group III-Vcompound, such as Indium Phosphide (InP). The plurality of MZMs may haveall of their optical input ports and optical output ports positioned onone face of the modulation block, and all of their electrical tracespositioned on a different face of the modulation block. In anembodiment, a waveguide connected to each optical output port may befolded or bent, enabling an incoming electrical signal to propagate in adirection substantially parallel to the optical signal being modulated.Moreover, the modulation block may be coupled via an optical interfaceto a planar lightwave circuit (PLC), which may comprise a passiveoptical network component. Further, the PLC may be monolithically grownon a PLC carrier based on a silica-on-silicon (SOC) chip. The modulationblock may be coupled to the PLC carrier via electrical, mechanical,and/or thermal interfaces. In an embodiment, the modulation may bebonded onto the PLC carrier using a flip-chip method.

FIG. 1 is a schematic diagram of an embodiment of a modulation block 100comprising an MZM array. The term “block” herein may refer to an object,device, unit, or entity that is separate from other objects (at least ata time when the object is first fabricated), thus merely a section orportion of the object (e.g., a left section or a right section as may bearbitrarily defined) may not be regarded as a block. FIG. 1 may beconsidered a top view of the modulation block 100, assuming that acarrier or platform is situated underneath the modulation block 100. Asused herein, “top”, “bottom”, “front”, “back”, “left”, “right”, “inner”,“outer”, or any other term that references a relative position is withrespect to the perspective view referenced and does not mean to implythat a device is restricted to only one orientation.

The modulation block 100, sometimes also referred to as an activecircuit component, may comprise one or more MZMs 110 or other type ofoptical modulators, each having an optical input port 102 and an opticaloutput port 107. Consider, for example, one MZM 110 located on top ofthe MZM array, with the premise that all other MZMs 110 may beconfigured similarly. The MZM 110 may be used to modulate the phaseand/or amplitude of an optical signal. Specifically, the MZM 110 may beconfigured to receive an optical input signal 101 via an optical inputport 102. The optical input signal 101 may be an un-modulated photonicbeam, such as laser light, or a pre-modulated signal from a laser orother photonic network component. The optical input signal 101 may passalong a waveguide 103 through the optical input port 102. Then, theoptical input signal 101 may be split into two parts or portions, e.g.,by a 1:2 multi-mode interference (MMI) splitter. A first portion and asecond portion may then pass through a waveguide 104 and a waveguide105, respectively.

In addition, the MZM 110 may also be configured to receive an electricalinput signal 120, which may be a radio frequency (RF) signal. Theelectrical input signal 120 may be delivered via a transmission line 122to an electrode 124, which may be positioned beneath and close to thewaveguide 105. The electrode 124 may reside on the surface of themodulation block 100 (e.g., having roughly the same length as thewaveguide 105), and the transmission line 122 may reside on a carrierunderneath the modulation block 100. The transmission line 122 may becoupled to the electrode 124 via solder or other suitable electricalattachment. Although the electrode 124 and the transmission line 122 areportrayed as beside the waveguide 105 for visual purposes, in practicethey may be positioned substantially underneath (e.g., verticallyoverlapping with) the waveguide 105. One skilled in the art willrecognize that, for proper transfer of the electrical input signals 120,the electrode 124 may be further connected to an electrical output,which may be connected to a common ground or to another component. Toreduce visual complexity, in FIG. 1 and all subsequent figures, unlesspointed out otherwise, locations of the electrical outputs, the commonground, and transmission lines coupled to the ground have been omitted.

In operation, the electrical input signal 120 may induce a change in therefractive index of the waveguide 105. Consequently, as the secondportion of the optical input signal passes through the waveguide 105,its phase may be altered. Then, the second portion of the optical inputsignal may be combined with the first portion of the optical inputsignal in the waveguide 104, e.g., by a 2:1 MMI coupler or combiner. Thetwo portions may combine to form an optical output signal 108 that ismodulated by the electrical input signal 120. Depending on theelectrical input signal 120, the optical output signal 108 may have anamplitude higher than, equal to, or lower than the optical input signal101. The optical output signal 108 may then propagate in a waveguide 106to the optical output port 107 and be transmitted to other components.

Although the waveguides 103, 104, 105, and 106 are described above asseparate waveguides, it should be understood that they may also beconsidered different sections or branches of one waveguide. Moreover,the waveguides 103, the waveguide 106, and the electrode 124 may beconsidered part of the MZM 110 or coupled to the MZM 110. Further,although only one arm (e.g. the waveguide 105) of the MZM 110 ismodulated in FIG. 1, one skilled in the art will recognize that,depending on the application, both arms of the MZM 110 may be modulatedas well. For example, two electrical input signals fed to the two armsmay have opposite polarity, such that the modulation effect may bestrengthened.

Regarding the modulation block 100 as a whole, the multiple MZMs 110 maybe positioned substantially in parallel to form an array configuration.In an embodiment, all of the optical input ports 102 and all of theoptical output ports 107 may be positioned on a first face or side 130of the modulation block 100. As shown in FIG. 1, the face 130 may beconsidered a left face of the modulation block 100. For each MZM 110,since the optical input port 102 and the optical output port 107 arepositioned on the same side, a waveguide needs to be redirected or benteither before entering or after exiting the two arms of a MZM. As shownin FIG. 1, the waveguides 106 connected to the optical output ports 107are bent (e.g., after existing two arms), which may be referred tosometimes as a lumped configuration. In an embodiment, the waveguide 106may be bent at least 120 degrees, at least 135 degrees, or at least 160degrees (e.g., 180 degrees to reverse direction). Further, the face 130of the modulation block 100 may comprise a center portion 132 and atleast one outer portion 134 (two are shown in FIG. 1). As shown in FIG.1, the optical input ports 102 are positioned in the inner portion 132and the optical output ports 107 are positioned in the two outerportions 134. Alternatively, all of the optical output ports 107 may bepositioned in only one outer portion 134. For example, the centerportion 132 may be a top part of the face 130, and the outer portion maybe a bottom part of the face 130.

In an embodiment, all electrical traces or patterns, such as electricalinput ports, transmission lines, electrodes, electrical outputs, may bepositioned on a second side or face of the modulation block 100. Thesecond side face may be considered a bottom face of the modulation block100. Positioning the electrical traces on a different side of themodulation block 100 from the optical input/output ports may reduce theneed for complex routing of electrical traces, since the electricaltraces may not vertically overlap with optical waveguides anymore.Specifically, such positioning allows for simple electrical signalrouting which reduces complexity and allows for relatively uniformelectrical path length. Uniform electrical path length further reducescomplexity by ensuring relatively uniform electrical signal propagationdelay, thereby reducing or eliminating the need for devices tocompensate for differing propagation delay on different electricalpaths.

Overall, the modulation block 100 is configured to receive optical inputsignals 101 through optical input ports 102, modulate intensity of thesignals via the MZMs 110, redirect those signals about 180 degrees, andtransmit the optical output signals 108 through the optical output ports107 to other components or devices. One skilled in the art willrecognize that, while four MZMs 110 are shown in the optical devicearray, any number of MZMs 110 may be used. For photonic integrationapplications, MZMs 110 may often be employed in multiples of two,multiples of four, multiples of eight, and so forth.

Isotropic materials are employed to allow optical signals to be bent atrelatively sharp angles in a small area on a circuit component. Usingthis property, the devices disclosed herein have both optical inputs andoptical outputs positioned on the same side of the circuit component.The optical inputs may be modulated using MZMs to create optical signalswhich may be multiplexed with other signals or otherwise employed byother related components. An optical medium used to channel thewaveguides discussed herein may comprise any appropriate isotropicmaterial. In an embodiment, the optical medium may comprise a GroupIII-V compound, such as Indium Phosphide (InP) or Gallium Arsenide(GaAs). Compared to MZMs based on lithium niobate, which is a materialcommonly used to construct MZMs, MZMs based on a Group III-V compoundmay possess some unique features. For example, the size or dimensions ofthe MZM may be constructed smaller, which may be desirable forlarge-scale photonic integrations. MZM made of InP may be especiallysmall, due to its use of a quantum-confined Stark effect. Exemplarydimensions (length, width, or height) of the modulation block 100, whichhas four lumped MZMs, are about a few (e.g., 2-10) millimeters. Notethat any other size may be realized depending on the application andfabrication technologies. For another example, MZMs based on a III-Vcompound may be built into a basic building block with a single opticalinterface, including optical inputs and outputs, for easy alignment inhybrid photonic integration. The single-interface block may offer a highyield when integrated onto a PLC.

The configuration of the modulation block 100 and its variations may beattractive in coherent communications and high-density multi-channelDWDM applications. For example, in polarization-multiplexed quadraturephase-shift-keying (PM-QPSK) applications, each laser may require fourparallel MZMs, with two for the X-polarization and two for theY-polarization. Each polarization has one MZM for in-phase modulationand one MZM for quadrature modulation. For another example, inhigh-port-density and massively-parallel DWDM applications, such asphotonic integrated circuit (PIC) and photonic integrated device (PID),increasing data rate per-wavelength beyond 10 gigabits per second (Gbps)as well as their reach may require the integration of a large number ofMZMs in parallel. The embodiments discussed herein may be used to createa PID and/or a PIC. A PID may be a PIC that is integrated in anon-monolithic fashion. Basic building blocks, such as the modulationblock 100, may offer an excellent foundation for PIC/PID to grow.

A method is described herein for hybrid integration of modulationblocks, which may be made of compound materials (e.g., InP), with asilica-on-silicon (SOS) carrier. The SOS carrier may also serve as apassive part of the integrated device. The integrated device may work asany appropriate optical device, such as a coherent modulator, a coherenttransmitter, a parallel on-off keying (OOK) modulator array, a parallelOOK transmitter array, or combinations thereof. In an embodiment, aflip-chip approach may be used for the hybrid integration.Alternatively, a Butt joint method may be used, which may achieve activealignment of the modulation block relative to the SOS carrier.

FIG. 2 is a schematic diagram of an embodiment of an assembly 200comprising a passive optical network component 210 coupled to amodulation block 230 via an optical interface. Specifically, the passiveoptical network component 210 may be monolithically grown on a PLCcarrier 250. Note that other type of optical components may also begrown on the PLC carrier 250 and coupled to the modulation block 230.The passive optical network component 210 may be a PLC or part of a PLC.Alternatively, sometimes both the passive optical network component 210and the PLC carrier 250 may be considered a PLC. The modulation block230, e.g., made of InP, may be bonded to the PLC carrier 250 (e.g., viaflip-chip bonding). An electrical interface may be formed between themodulation block 230 and the PLC carrier 250.

The passive optical network component 210 is configured to performprocessing and/or transmission of an optical signal. The passive opticalnetwork component 210 may comprise one or more inputs 203 on a first endand one or more corresponding optical output ports 205 on a second end(only one 203 and one 205 labeled for illustration). The passive opticalnetwork component 210 may receive an optical or other input, perform anyappropriate processing, and transmit one or more optical signals to theoptical output ports 205. The optical output ports 205 may be alignedwith or coupled to the optical input ports 102 of the modulation block230. The optical signals may then be received by the optical input ports102, modulated by MZMs in the modulation block 230, and transmitted viathe optical output ports 107 back to the passive optical networkcomponent 210.

The passive optical network component 210 may further comprise one ormore optical input ports 207 on the second end and one or morecorresponding output ports 209 on the first end. The optical input ports207 may be aligned with the optical output ports 107 and accept opticalsignals from the optical output ports 107. After processing by thepassive optical network component 210, an optical input signal may beconverted to a processed signal (optical or other type), which may thenbe transmitted to external components via the output ports 202. Oneskilled in the art will recognize that the passive optical networkcomponent 210 or other components may be optically coupled to thevarious embodiments of the modulation block 230 by butt joint, flipchip, or other methods. One skilled in the art will recognize that it ispossible to integrate other optical components such as semiconductoroptical amplifiers (SOA) with the various embodiments discussed herein.The SOAs may be used to boost output signal intensity to or from themodulation block 200. The SOA may be located along an optical input port102 waveguide, optical output port 107 waveguide, or both.

Various aspects of the modulation block 230 (e.g., working principle ofMZMs, material, and size) may be substantially similar to the modulationblock 100, thus in the interest of conciseness, further descriptions mayfocus on the aspects that are different. Unlike the modulation block 100which is configured to redirect optical input signals after modulation,the modulation block 230 is configured to redirect optical input signalsprior to modulation (sometimes referred to as a folded configuration).In the modulation block 230, the waveguide connected to the opticalinput ports 102 is bent or folded at least 120 degrees, at least 135degrees, or at least 160 degrees (e.g., about 180 degrees to reversedirection). In other words, a light path of optical input signals areredirected at least 120 degrees, at least 135 degrees, or at least 160degrees.

Consider the operation of one MZM as an example, with the premise thatother MZMs may be similarly configured. In operation, an electricalinput signal (e.g., a RF signal) may feed into the wire-bonding pad 240,which is connected to a transmission line 242. The electrical inputsignal may propagate along the transmission line 242 (e.g., underneaththe MZM modulation arm) with one or more ground lines, such as a firstground line 244 and a second ground line. Electrical connections areonly shown for one MZM, but it should be understood that the other MZMsalso have similar corresponding electrical connections. Since in thefolded configuration, the optical input signal has already beenredirected when arriving at the MZM modulation arm (verticallyoverlapping the transmission line 240), the optical input signal maypropagate in the same direction as the electrical input signalpropagates in the transmission line 240. For high-speed systems, themodulation block 230 may be beneficial because the MZMs are arranged ina way such that the optical signals inside the MZMs propagate in thesame direction as the electrical input signals, thereby increasing theeffective interaction length between the signals for efficienthigh-speed modulation. The traveling-wave approach used in themodulation block 210 may provide relatively higher modulation bandwidththan the lumped modulation block 100 shown in FIG. 1.

To properly operate a MZM using a traveling-wave approach (e.g., asshown in FIG. 2), it may be desirable to have minimal RF reflection inthe electrical circuit. A high amount of RF reflection may destabilizethe operation of a RF signal source providing the RF signal and/orcorrupt signal fidelity by adding reflected out-of-phase artifacts tothe signal path, thereby reducing an operation bandwidth (e.g., afrequency range of optical modulation). In use, RF reflections in ahigh-speed electro-optic device may be caused by an impedance mismatch.In general, an impedance mismatch may indicate that a load (e.g., thetransmission line 240) has an impedance different from the electricalsignal source. Note that the signal source may include not only the RFsignal source, but also connector(s) and other electrical signalsources, but not the load (e.g., the transmission line 240). Thus, tominimize the RF reflection in each MZM, RF termination resistors withappropriate values may be incorporated to the assembly 200.

Termination resisters may be deposited and patterned onto the surface ofeither the modulation block 230 or the PLC carrier 250. In practice, itmay be undesirable to deposit the termination resistors on themodulation block 230. For example, a termination resistor may generateadditional heat as modulation current flows in it, leading to atemperature increase in the modulation block 230. Since the performanceof the modulation block 230 may strongly depend on its temperature, thetemperature increase may cause decrease in performance. For anotherexample, depositing and patterning termination resistors on the surfaceof the modulation block 230 may require additional patterning masks(e.g., lithography masks), increase architecture complexity, andincrease manufacture cost. Moreover, the termination resistors on themodulation block 230 may also take additional space in a highlyintegrated MZM array.

FIG. 3 is a schematic diagram of an embodiment of an assembly 300, whichmay be substantially similar with the assembly 200 in FIG. 2, exceptthat FIG. 3 highlights RF connections of the assembly. In an embodiment,termination resisters 310 are deposited on the PLC carrier 250, whichresides underneath the modulation block 230. For each MZM, there may bea termination resistor 310 located on the PLC carrier 250. Thetermination resistor 310 may be connected to a bonding pad 312, which isalso located on the PLC carrier 250. The bonding pad 312 may be coupledvia a solder 314 to a transmission line 316, which is located on themodulation block 230. As shown in FIG. 3, there may be another bondingpad 318 located on the PLC carrier 250 and coupled to the transmissionline 316 via another solder 320. The solders 314 and 320 may compriseany fusible metal or metallic alloy used to join metal work pieces andhaving a melting point below that of the work piece(s). Exemplarysoldering materials include, tin, copper, silver, bismuth, indium, zinc,antimony, and any combination thereof. Part of the bonding pad 318 mayalso serve as a wire bond or wire-bonding pad 322, through which anelectrical signal travels to reach the transmission line 316. Theelectrical signal may propagate along the transmission line 316, then goto the termination resistor 310, and eventually to a common groundcoupled to the termination resistor 310. Although the transmission line316 is portrayed as beside the modulation MZM arm for visual purposes,in practice, the transmission line 316 may be positioned substantiallyunderneath (e.g., vertically overlapping with) the waveguide 105. In anembodiment, the impedance of each termination resistor 310 is designedto match the impedance of the RF signal source. As a result, anyremaining RF power in the back end of the electrical circuit may beminimized or eliminated, without causing noticeable perturbation to theoperation. Moreover, the termination resistor 310 may be made by anysuitable technique, e.g., by thick-film or thin-film technologies.

Further, in hybrid integration, the modulation block 230 may be placedat an edge of the PLC carrier 250, that is, not on top of any opticalcomponent or PLC. For high-speed modulation, e.g., at 25 gigahertz(GHz), it may be desirable to minimize the length of a RE connectionbetween the wire bond 322 and the wall of a device package. The devicepackage encompasses the assembly 300 and connects to external circuitry,e.g., via pins. As shown in FIG. 3, the wire bond 322 may be positionedat or near the edge of the modulation block 230 (away from the passiveoptical network component 210). Thus, the RF connection to the wall ofdevice package may be made as short as possible.

In an embodiment, the modulation block 230 and the PLC carrier 250 maybe integrated using a flip-chip bonding method. FIG. 4 illustrates anelectrical interface and an optical interface between the modulationblock 230 and the PLC carrier 250. FIG. 4 may be considered a front sidecross-sectional view of the assembly 300. The wire bond 322 may serve asa contact point to a RF input signal. The wire bond 322 may be part ofthe bonding pad 318 or may be connected to the bonding pad 318, e.g.,via a gold strip. The wire bond 322 and the bonding pad 318 may be twoends of a single metal pad (e.g., made of gold). The bonding pad 318 iscoupled to one end of the signal trace or transmission line 316 on themodulation block 230 via the solder 320. Another end of the transmissionline 316 may be coupled to the bonding pad 312 via the solder 314. Thebonding pad 312 is connected to the termination resister 310, which maybe coupled to a common ground via a ground line (not shown in FIG. 4).During modulation, a RF signal may first be fed from a wall of thepackage body via the wire bond 322 to the transmission line 316. Thetransmission line 316 may be positioned underneath and close to a core330, which is an arm of a MZM. Optical signals passing through the core330 may be modulated in intensity and/or phase. In use, the transmissionline 316 may comprise a relatively wider portion, which serves as abonding pad to form an electrical connection between the modulationblock 230 and the PLC carrier 250. The electrical connection may beachieved by solder jointing of bonding pads located on both themodulation block 230 and the PLC carrier 250. Any remaining RF power maybe largely absorbed by the termination resistor 310 on the PLC carrier250, as the electrical path may be designed to minimize impedancemismatch and optimize electrical signal propagation. It should beunderstood that, in some embodiments (e.g., the lumped configuration inFIG. 1), no termination resister may be needed in the electricalcircuit. Although construction of an assembly may be simpler in thiscase, the modulation bandwidth may be relatively narrower.

The optical interface between the modulation block 230 and the passiveoptical network component 210 may be formed by aligning the core 230with a core 240. The cores 230 and 240 may be waveguides describedpreviously. During fabrication of the device assembly, to verticallyalign the two cores, the PLC carrier 250 may be partially etched so thatthe waveguide cores 230 and 240 in the modulation block 230 and thepassive optical network component 210 may be aligned at an equal height,as shown in FIG. 4. As the PLC carrier 250 may be a silica-on-silicon(SOS) carrier, it may have silica (e.g., SiO₂) on top and silicon atbottom. Etching does not have to reach the silicon layer.

FIG. 5 is another schematic diagram showing an interface between themodulation block. 230 and the PLC carrier 250 formed via flip-chipbonding. FIG. 5 may be considered a right side cross-sectional view ofthe assembly 300 (e.g., a view parallel to the optical interface in FIG.4). As shown in FIG. 5, the vertical alignment may be obtained through anumber of protruding features 510 on the PLC carrier 250, which may bereferred to herein as stoppers 510. During fabrication, the stoppers 510may be created via silica etching on the PLC carrier 250. For a stopper510, its two sides may have different etching depths, e.g., with oneside relatively shallower and another side relatively deeper. Theshallower side may be designed to support electrical traces or patterns.For example, a shallower side may support a bonding pad 318, which maybe connected to a transmission line 316 via a solder 320. As the bondingpad 318 may be made of gold, which may have good adhesion to silica butpoor adhesion to silicon, it may be desirable to leave a layer of silicabetween the bonding pad 318 and the silicon layer of the PLC carrier250. On the other hand, the deeper side may be etched all the way to thesilicon layer and may be designed to provide mechanical support for themodulation block 230. For example, a solder 520 may couple two bondingpads 530 and 540 together forming a mechanical and thermal interface.The bonding pad 540 may be deposited on the silicon surface, thus thebonding pad 540 may comprise metals which has good adhesion to silicon.In an embodiment, mechanical coupling between the modulation block 230and the PLC carrier 250 may be achieved in area where there is no directcontact to any MZM arm or electrode. This design may minimize or reduceany stress/strain induced birefringence as a result of the bonding,while keeping the mechanical/thermal characteristics of the interfaceuncompromised.

FIG. 6 is another view taken along the stopper plane shown in FIG. 5. Asshown in FIG. 6, vertical alignment between the modulation block 230 andthe PLC carrier 250 may be obtained through a number of stoppers 510(only one shown in FIG. 6). The stoppers 510 may extend from an opticalinterface between the waveguide cores 330 and 340 to the other side ofthe modulation block 230. In a PLC wafer etching process, the depth ofsilica etching may be precisely controlled. The stoppers 510 may bepositioned a few (e.g., 2-10) micrometers below the center of thewaveguide core 330 in the modulation block 230. To align the waveguidecore 330 with another waveguide core 340 (e.g., in the passive opticalnetwork component 210 or in another modulation block), the depth of thewaveguide core 330 (e.g., distance of the core 330 to the bottom surfaceof the modulation block 230 in FIG. 6) may determine the height of thestoppers 510. Furthermore, during flip-chip bonding, the modulationblock 230 may be pressurized until it rests on the stoppers 510. Atypical precision of passive alignment in the vertical direction usingthe stoppers 510 may be less than 1 micrometer.

Horizontal alignment of the modulation block 230 with respect to the PLCcarrier 250 may be accomplished using markers on both the modulationblock 230 and the PLC carrier 250. The markers on the modulation block230 may be generated during fabrication of the core 330 (etching insideInP), and the markers on the PLC carrier 250 may be generated duringfabrication of the stoppers 510. Markers on both devices may be placednear the optical interface area for easy alignment. In addition, to anextent, soldering may also help horizontal alignment of the PLC carrier250 and the modulation block 230, since horizontal movement may bedriven by a surface tension force in an effort to minimize the surfacearea to reaching the lowest total surface energy of the assembly.

FIG. 7 is another view taken along the solder plane shown in FIG. 5. Asshown in FIG. 7, the modulation block 230 may be mechanically heldtogether with the PLC carrier 250 by the solder 520. Specifically, afirst bonding pad 530 patterned on the modulation block 230 may becoupled with a second bonding pad 540 patterned on the PLC carrier 250via the solder 520. In an embodiment, to provide good thermalconductivity between the modulation block 230 comprising InP and the PLCcarrier 250 comprising silicon, the silica may be etched all the way tothe silicon layer, so that no silica layer remains. Since the thermalconductivity of silicon is better than silica, this configuration mayprove useful in case temperature control of the assembly is needed.Otherwise, silica etching may stop at any level (e.g., the same levelwith areas supporting the RF electrode connections, along the MZM exitplane, as shown in FIG. 5).

Flip-chip bonding of the modulation block 230 on the PLC carrier mayoffer good thermal stability in comparison to other bonding methods orapproaches. For example, although a Butt joint method may be used toform the optical interface via active alignment, as the modulation block230 may need to be repeatedly detached from the PLC carrier 250 duringalignment, the Butt joint method may be thermally less stable comparedto flip-chip bonding. The thermal stability may be important in somedevices, e.g., where a precise path difference between two MZMs must bekept to maintain a fixed phase difference, such as in QPSK modulation.

Mode convertors may sometimes be needed for both the modulation block230 and the PLC carrier 250, in order to ensure good optical coupling atthe optical interface. FIG. 8 is a schematic diagram showing an opticalinterface between a first waveguide 810 and a second waveguide 820. Thewaveguide 810 may be inside a modulation block, and the waveguide 820may be inside another modulation block or an optical network component.The waveguides 810 and 820 may be substantially similar with waveguidesdescribed previously, except that the direction of the waveguide 820 istilted to create a horizontal misalignment with the waveguide 810, asshown in FIG. 8. The misalignment causes a direction of the waveguide820 to be different from a direction of the waveguide 810. Themisalignment angle may be greater than zero but less than 45 degrees,less than 30 degrees, or less than 15 degrees. As a result, an exitfacet of the waveguide 820 may no longer face perpendicularly to an exitfacet of the waveguide 810. This configuration may help reduce orminimize optical reflection from the etched PLC facet at the opticalinterface. For example, when an optical signal travels from thewaveguide 810 to the waveguide 820 through an air gap, the opticalsignal may be partially reflected upon entrance into the waveguide 820,with horizontal tilting, the optical reflection at the etched interfacemay be directed away from the optical path.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R_(l), and an upperlimit, R_(u), is disclosed, any number falling within the range isspecifically disclosed. In particular, the following numbers within therange are specifically disclosed: R=R_(l)+k*(R_(u)−R_(l)), wherein k isa variable ranging from 1 percent to 100 percent with a 1 percentincrement, e.g., k is 1 percent, 2 percent, 3 percent, 4 percent, 5percent, . . . , 70 percent, 71 percent, 72 percent, . . . , 95 percent,96 percent, 97 percent, 98 percent, 99 percent, or 100 percent.Moreover, any numerical range defined by two R numbers as defined in theabove is also specifically disclosed. The use of the term “about”means±10% of the subsequent number, unless otherwise stated. Use of theterm “optionally” with respect to any element of a claim means that theelement is required, or alternatively, the element is not required, bothalternatives being within the scope of the claim. Use of broader termssuch as comprises, includes, and having should be understood to providesupport for narrower terms such as consisting of consisting essentiallyof, and comprised substantially of. Accordingly, the scope of protectionis not limited by the description set out above but is defined by theclaims that follow, that scope including all equivalents of the subjectmatter of the claims. Each and every claim is incorporated as furtherdisclosure into the specification and the claims are embodiment(s) ofthe present disclosure. The discussion of a reference in the disclosureis not an admission that it is prior art, especially any reference thathas a publication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it may be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: a modulation blockcomprising a plurality of modulators, wherein the plurality ofmodulators comprises a first modulator at least partially folded withina second modulator, wherein each of the plurality of modulatorscomprises an optical input port and an optical output port, wherein allof the optical input ports and all of the optical output ports arepositioned on one face of the modulation block, wherein the firstmodulator comprises a first optical input port and a first opticaloutput port and the second modulator comprises a second optical inputport and a second optical output port, and wherein partially foldedmeans that the first optical input port, the first optical output port,or both are located between the second optical input port and the secondoptical output port on the one face.
 2. The apparatus of claim 1,wherein the modulators are Mach-Zehnder modulators (MZMs).
 3. Theapparatus of claim 2, wherein each of the MZMs further comprises IndiumPhosphide (InP).
 4. The apparatus of claim 2, wherein each of the MZMsfurther comprises a waveguide connected to the optical input port, andwherein the waveguide comprises a curved section.
 5. The apparatus ofclaim 2, wherein each of the MZMs further comprises one or moreelectrical connections, and wherein all of the electrical connectionsare positioned on another face of the modulation block.
 6. An apparatuscomprising: a modulation block comprising: a first plurality of markers;and one or more Mach-Zehnder modulators (MZMs), wherein each MZM iscoupled to an optical input port, an optical output port, and at leastone electrical input, wherein all of the optical input ports and all ofthe optical output ports are positioned on a first side of themodulation block, and wherein all of the electrical inputs arepositioned on a second side of the modulation block that is parallel tothe first side; an optical interface; a planar lightwave circuit (PLC)carrier comprising: a second plurality of markers; and a plurality ofstoppers comprising first portions and second portions, wherein thefirst portions are etched and configured to provide electricalinterfaces with the modulation block, and wherein the second portionsare etched deeper than the first portions and configured to providethermal interfaces and mechanical interfaces with the modulation block;and a PLC monolithically grown on the PLC carrier and optically coupledto the modulation block via the optical interface.
 7. The apparatus ofclaim 6, wherein the PLC comprises a passive optical network component.8. The apparatus of claim 6, wherein the modulation block is coupled tothe PLC carrier.
 9. The apparatus of claim 8, wherein no electricalinput overlaps any optical component positioned on the PLC carrier, andwherein the PLC is further coupled to the modulation block via flip-chipbonding.
 10. The apparatus of claim 8, wherein at least one of theoptical output ports is offset from a PLC optical input port by an anglegreater than zero and less than 30 degrees.
 11. The apparatus of claim8, wherein the modulation block comprises a first waveguide, wherein thePLC comprises a second waveguide coupled to the first waveguide via theoptical interface, and wherein the plurality of stoppers are configuredto align the modulation block such that the first waveguide and thesecond waveguide have equal vertical positions at the optical interface.12. The apparatus of claim 11, wherein the first plurality of markersand the second plurality of markers are configured to align themodulation block to the PLC carrier such that the first waveguide andthe second waveguide have equal horizontal positions at the opticalinterface.
 13. The apparatus of claim 11, further comprising at leastone additional modulation block coupled to the PLC carrier.
 14. Theapparatus of claim 1, further comprising: a planar lightwave circuit(PLC) carrier; a first termination resistor deposited on the PLC carrierand associated with the first modulator; and a second terminationresistor deposited on the PLC carrier and associated with the secondmodulator.
 15. The apparatus of claim 1, further comprising a planarlightwave circuit (PLC) carrier, wherein the PLC carrier comprises afirst plurality of markers and the modulation block comprises a secondplurality of markers.
 16. The apparatus of claim 15, further comprisinga PLC monolithically grown on the PLC carrier, wherein the PLC comprisesa first waveguide and the modulation block comprises a second waveguide.17. The apparatus of claim 16, wherein the first plurality of markersand the second plurality of markers are configured to align themodulation block to the PLC carrier such that the first waveguide andthe second waveguide have equal horizontal positions.
 18. The apparatusof claim 6, wherein the one or more MZMs comprises a first MZM, andwherein the apparatus further comprises a first termination resistordeposited on the PLC carrier and associated with the first MZM.
 19. Theapparatus of claim 18, further comprising: a bonding pad coupled to thefirst termination resistor; a transmission line coupled to themodulation block; and a solder coupling the bonding pad to thetransmission line.
 20. The apparatus of claim 6, wherein the firstportions are etched to silicon dioxide (SiO₂) layers and the secondportions are etched to silicon (Si) layers.